This invention concerns a cryopump comprising pump surfaces held at different temperatures during operation and situated in a housing with a flange for connecting the pump to a vacuum chamber.
Cryopumps for the production of a high and ultrahigh vacuum are generally operated using a two-stage refrigerator comprising a two-stage refrigeration head. Cryopumps have three pump surface areas designed to adsorb various types of gas. The first surface area is thermally well linked to the first stage of the refrigeration head and attains a temperature of about 80 K, depending on the type and power rating of the refrigerator. Commonly, a thermal radiation shield and a baffle are assigned to these surface areas. These components protect the pump surfaces at lower temperatures against being exposed to entering thermal radiation. Moreover, they form the pump surfaces of the first stage, preferably serving the purpose of adsorbing relatively easily condensable gases, like hydrogen and carbon dioxide, by way of cryocondensation.
The second pump surface area is thermally well linked to the second stage of the refrigeration head. During operation of the pump this stage attains a temperature of about 20 K and less. The second surface area is preferably employed to remove gases which only condense at lower temperatures, like nitrogen, argon or alike by way of cryocondensation, as well as trapping lighter gases like H.sub.2 or He in a majority of the aforementioned condensable gases. The third pump surface area also attains the same temperature as the second stages of the refrigeration head (in the case of a refrigeration head having three stages correspondingly lower) said pump surface being covered by an adsorbing material. Chiefly the process of cryosorption of lighter gases like hydrogen, helium and alike takes place on these pump surfaces.
When employing cryopumps in the areas of coating technology, sputter processes or ion implantation, the suction performance for water vapour which is restricted by the size of the high vacuum flange and the related pump surfaces will no longer be sufficient. In such cases, the additionally required pumping performance for water vapour is attained by further pump surfaces which are installed in the process chamber. These pump surfaces are cooled with liquid nitrogen (MeiBner trap), with Freon, with Freon substitute machines or single-stage refrigerators like those operating according to the Gifford-McMahon principle. Cooling the additionally required pump surfaces with liquid nitrogen is relatively costly; handling of the liquid nitrogen is involved. The Freon coolers are large and expensive; even the Freon substitutes may not be employed without reservations as to the environment. Finally, also additional refrigerators are involved and expensive.